Priority and associativity are two of the important concepts in Python expressions. They determine which part of the expression is executed first.
The so-called priority is which operator is executed first when multiple operators appear in an expression at the same time.
For example, for the expression a + b * c, Python will first calculate the multiplication and then the addition; b * c will result in 8 and a + 8 will result in 24, so the final value of d will also be 24. Calculate * before calculating +, indicating that * has higher priority than +.
Python supports dozens of operators and is divided into nearly twenty priorities. Some operators have different priorities, and some operators have the same priority. See the table below.
|Operator Description||Python Operators||Priority||Cohesiveness|
|Parentheses||( )||19 (Highest)||No|
|Index operator||x [i] or x [i1: i2 [: i3]]||18||Left|
|Symbolic operator||+, -||14||Right|
|Multiply and divide||*, /, //, %||13||Left|
|Addition and subtraction||+, -||12||Left|
|Comparison operator||==, !=, >, >=, <, <=||7||Left|
|is operator||is, is not||6||Left|
|in operator||in, not in||5||Left|
|Comma operator||exp1, exp2||1 (Lowest)||Left|
As a result of the operator precedence in above table, we try to analyse the results of the following expressions:
4 + 4 << 2
The priority of + is 12, the priority of << is 11, the priority of + is higher than <<, so execute 4 + 4 first to get the result 8 and then execute 8 << 2 to get the result 32, which is also the final result.
For the case that is not easy to determine the priority, we can add () to the subexpression, which is written as follows:
(4 + 4) << 2
This seems obvious and is not easy to cause misunderstanding.
Of course, we can also use () to change the execution order of the program, such as:
4 + (4 << 2)
Then execute 4 << 2 to get the result of 16 and then execute 4 + 16 to get the result 20.
Although there is a precedence relationship for Python operators, but it is not recommended to rely too much on the precedence of the operators, which will make the program less readable. Therefore, it is better don't write an expression too complicated. If an expression is too complicated, you can try to write it separately.
Don't rely too much on the precedence of the operators to control the execution order of the expressions, so readability is too poor, you should use () to control the execution order of the expressions whenever possible.
When multiple operators appear in an expression, Python will compare the precedence of each operator, and executes them from high to low order. When operators with the same precedence are encountered, then according to the cohesiveness degree to decide which operator to execute first: If it is left cohesiveness, the left operator is executed first, and if it is right cohesiveness, the right operator is executed first.
More Tutorials:Python Installation - Linux (Ubuntu)
More Python Exercises:Python String Exercises